In modern automotive engines, regenerative pumps are being used for their fuel pumps in an increased number not only because of their compactness in size but also because of their favorable properties to produce high pressure heads and low flow rates. In a pump of this structure, in particular, a pump which is sometimes referred to as peripheral type because the pump rotor thereof consists of a disc having a plurality of vanes along its periphery, since the fuel inlet is relatively small and the fuel is introduced perpendicularly in relation to the outlet passage, the vapor tends to be separated from the fuel and cavitation tends to occur with the result that the vapor locking takes place and the efficiency of the pump is severely impaired.
Therefore, a number of proposals have been made in the past to remove vapor from the fuel inlet of a pump of this type. For instance, Japanese Patent Publication No. 46-26443 which corresponds to U.S. patent application No. 645,150 filed June 12, 1967 discloses a vent hole (denoted with numeral 86) provided in the arcuate passage defined alnng the pump wheel of a peripheral type pump. The vent hole is located adjacent to the root end portions of the vanes of the pump wheel and is preferred to be located at a relatively upper part of the pump when the pump is in use for effective removal of vapor from the fuel.
In Japanese Patent Laid-Open Publication No. 60-79193 which also discloses a regenerative fuel pump, the arcuate fuel passage which is defined along the oute periphery of the pump rotor is broadened at its portion adjacent to the inlet port of the fuel pump and a vapor vent opening is provided at a terminal end of this broadened part of the fuelppassage. The vapor contained in the fuel is separated from the liquid part of the fuel by the centrifgal force as the fuel flows through this broadened passage. However, since the effective length of the fuel passage which is defined along the outer periphery of the pump rotor is reduced by the length of the broadened passage, the pressure head of the pump is reduced. Also, since the vapor vent opening is provided at a location which is a certain distance from the inlet port, the vapor is compressed before it is released from the vent opening. Therefore, the pump work is wasted because of this compression of the vapor and the efficiency of the pump is thus impaired.